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Pfizer (PFE) shares were down more than 1% Monday afternoon despite the pharmaceutical giant's report Sunday of successful late-stage trial results for a closely watched experimental drug for rheumatoid arthritis. According to study data, the treatment, tasocitinib, reduced pain and inflammation and improved physical function in patients with moderately to severely active RA.

The world's largest drug company will soon lose patent protection for its cholesterol fighter Lipitor -- the world's best selling drug, with sales of $11.4 billion last year. Pfizer has been looking for new drugs to compensate for the loss of revenue that cheaper generics of Lipitor will bring, and tasocitinib has been one of the most promising in its pipeline.

If the oral pill lives up to expectations, it could supplant injectable rheumatoid arthritis drugs that currently generate $12 billion in yearly sales, according to Bloomberg. Analysts have been accordingly estimating multibillion dollar peak sales for tasocitinib.

Rheumatoid arthritis is a chronic inflammatory autoimmune disease that mostly affects hands and feet as the immune system mistakenly attacks the body's own tissues, damaging the lining of the joints. It can eventually result in bone erosion and joint deformity. The causes of the disorder remain unknown and there's no cure, but several treatment options exist. RA affects 1.3 million Americans -- and two to three times more women than men.
Significant Decreases in Symptoms

Belonging to a new class of drugs known as a JAK inhibitors, tasocitinib targets the intracellular signaling pathways involved in the inflammatory network. In the randomized trial of 611 patients, 65.7% of patients who received a dose of 10 milligrams of tasocitinib showed at least a 20% improvement in disease activity and symptoms after three months of treatment. Nearly 60% of patients who received 5 mg reached a similar improvement. This compares to 26.7% of those who received a placebo.

As for the much harder secondary target of achieving a 70% improvement, 20.3% of patients taking 10 mg achieved that goal, as did 15.4% of patients on the 5 mg dose. That compared with 5.8% on placebo.

Tasocitinib also achieved a second primary goal: improved physical function. Patients in the study were asked to gauge their improvement in physical functions on a 0-to-3 scale, in which 0 means normal function without any difficulty. Patients on the 10 mg dose reported a 0.57 scale drop, while patients on 5 mg reported a 0.5 decline. Placebo patients reported only a 0.19 decrease.

Still, tasocitinib didn't achieve a third primary goal: It demonstrated a numerically higher, but not statistically significant, improvement in disease remission rates at three months compared with a placebo.

The drug came with several adverse events, including higher LDL and HDL cholesterol and a significant decreases in neutrophil count -- a type of white blood cells that are part of the immune system. Also, 4.1% of patients reported serious adverse events during the six month trial.

The trial was the first of six late-stage studies that Pfizer is conducting for tasocitinib. If additional studies are positive, analysts think Pfizer might submit the drug for regulatory approval as early as next year, Dow Jones reports. Pfizer's most recent findings will be reported Wednesday at the American College of Rheumatology meeting in Atlanta.